Uterine hypertrophy is a physiological process occurring in pregnancy to accommodate fetal growth and the need for contraction at delivery. Growth factors, sex hormones and cytokines are known to stimulate uterine smooth muscle growth during normal pregnancy or fibrosis. Angiotensin II is a well documented vascular smooth muscle growth factor. However, the role of angiotensin II in uterine growth is not well defined, despite the fact that its concentration is increased and there is a significant switch in myometrial receptor subtype towards the type 1 angiotensin II receptor in pregnant women. NADPH oxidase isoforms, via generation of reactive oxygen species, have been shown to be responsible for the growth promoting effect of angiotensin II which is mediated by type 1 receptor in vascular smooth muscle and cardiac myocytes. We have identified several NADPH oxidase isoforms in human myometrium and in a human uterine smooth muscle cell line. We also found that a NADPH oxidase inhibitor blocked angiotensin ll-induced production of reactive oxygen species as well as uterine smooth muscle protein synthesis. Therefore, we hypothesize that angiotensin II plays an important role in myometrial growth during pregnancy via activation of NADPH oxidase, which is dependent on the type 1 angiotensin II receptor and downstream signaling pathways. We further hypothesize that NADPH oxidase is localized and activated in caveolae, the plasma membrane structures where several elements of a proposed signal transduction pathway were found. These include angiotensin II type 1 receptor, the catalytic unit of NADPH oxidase isoform 1 (Nox1), G proteins, and epidermal growth factor receptor. We will employ a combination of molecular biology, cell biology, and biochemical methods to study, in the uterine smooth muscle cell line, the following specific aims: 1) The role of angiotensin II in regulation of NADPH oxidase expression and activation in myometrial smooth muscle cells; and 2) The role of NADPH oxidase in angiotensin II receptor-mediated myometrial growth (hypertrophy) and signaling. Currently there is a great deal of interest in the role of reactive oxygen species in obstetric pathologies. It is known that many cases of preterm delivery are associated with infection where cytokines induce reactive oxygen species production. Excessive generation of reactive oxygen species contributes to contractile failure, rigor and calcium overload. Thus we predict that the angiotensin II-NADPH oxidase-reactive oxygen species pathway plays a role in labor process as well.